Geoelectric constraints on the Precambrian assembly and architecture of southern Laurentia

Much of the tectonic framework of southern Laurentia (Fig. 1A), particularly where Precambrian rocks are concealed beneath Phanerozoic cover (Fig. 1B), is informed by limited geochronologic and isotopic data (e.g., Bickford et al., 2015) and by inferences from geophysical techniques. The latter category has historically largely comprised potential field methods (gravity and magnetics) and seismic imaging. Analyses of the crustal magnetic field have in fact greatly shaped models of the tectonic evolution of Laurentia (e.g., Sims et al., 2008); however, magnetic methods are most sensitive to structures at upper- to mid-crustal depths, and they often provide virtually no information at lower-crustal depths due to the loss of magnetization at lithospheric temperatures above the Curie point (e.g., Bouligand et al., 2009). In contrast, passive-source seismic tomography models (e.g., Porritt et al., 2014; Schmandt and Lin, 2014) are often best resolved at lithospheric mantle depths and provide few constraints within the crustal column; surface-wave imaging (e.g., Porter et al., 2016; Shen and Ritzwoller, 2016) can provide information at crustal depths, albeit at limited spatial resolution. Although active-source seismic imaging (e.g., Allmendinger et al., 1987; Nelson et al., 1993; Magnani et al., 2004) provides a unique view through the crustal column, data coverage is sparse at the continent scale, and the technique is intrinsically limited in resolving structures within crystalline basement rocks due to ubiquitous diffractions and a general lack of continuous strongly reflective horizons.

With growing national-scale coverage, long-period magnetotelluric (MT) data provide an invaluable source of information with which to gain new insights into the Precambrian evolution of southern Laurentia and to complement other geological and geophysical studies. The lithospheric electrical conductivity images derived from these data provide meaningful structural and petrologic constraints through the entire lithospheric column and can be readily linked to surface geology, potential field data, and the full range of seismic results. Consequently, MT analyses bridge the gaps between these various geophysical approaches and provide perhaps the best images of mid- to lower-crustal structure.

Here, we present an updated and expanded (cf. Kelbert et al., 2019) three-dimensional electrical conductivity synthesis model that covers the majority (>80%) of the contiguous United States (CONUS). We highlight key large-scale conductivity anomalies within this synthesis model, and we also draw attention to regions where major anomalies are conspicuously absent. We then discuss the constraints that these observations place on Precambrian lithospheric structure in southern Laurentia and the consequent implications for competing tectonic models for this region.

In the context of continent-scale tectonics, MT imaging can provide structural, petrologic, and geodynamic constraints on terrane boundaries as well as physicochemical constraints on the crustal domains that constitute those terranes themselves. Here, we briefly review the causes and characteristics of electrical conductivity structures associated with suture zones, rift zones, and cratonic blocks. We also discuss the conditions under which electrical conductivity anomalies may not be associated with a given structure and the implications of such a lack of a conductivity signature. These considerations will form the basis for subsequent interpretations of the CONUS conductivity synthesis model.

Numerous studies worldwide have demonstrated that MT is particularly adept at imaging major terrane sutures (e.g., Park et al., 1991; Jones, 1993; Korja, 1993; Jones et al., 2002; Garcia and Jones, 2005; Rao et al., 2007; Spratt et al., 2009; Bologna et al., 2014; Yang et al., 2015; Padilha et al., 2017, 2019; Wunderman et al., 2018; Ye et al., 2019; Comeau et al., 2020). In conductivity images, these crustal boundaries are demarcated by quasilinear high-conductivity belts that stretch for hundreds or even thousands of kilometers along strike. The conductors often appear as vertical or steeply dipping planes at mid- to lower-crustal depths; in some cases, the conductors may extend into the upper crust and even to the surface. Such conductors have been observed to be associated with sutures that range in age from Paleoproterozoic (e.g., Bedrosian and Finn, 2021) to Cenozoic (e.g., Bedrosian et al., 2018), although the association with Precambrian structures is most widely observed.

Based on observations from the limited cases in which these conductors are known to extend to the surface, the high conductivity values are primarily the result of interconnected networks of graphite and/or sulfides within metasedimentary units that became trapped in these ancient zones of convergence (e.g., Boerner et al., 1996; Jones et al., 1997). The genetic model for formation of these conductors is shown in Figures 2A–2C. During terrane accretion, anoxic or euxinic (anoxic as well as sulfidic) passive margin and foreland basin deposits (e.g., black shales; generally, sediments rich in organic carbon and/or authigenic sulfides) become either incorporated into the accretionary wedge during convergence or subducted beneath the continental margin. Orogenic metamorphism within the wedge and high-pressure, low-temperature metamorphism within subducted sediments promote recrystallization of the original organic material into graphite, while deformation and fluid flow promote interconnection and concentration of both graphite and the sulfide minerals (Boerner et al., 1996; Jones et al., 1997; Jödicke et al., 2004; Wunderman et al., 2018). As these mineral phases are highly electrically conductive (bulk conductivity values of ~10¹ S/m are not uncommon for such metasedimentary rocks; Bedrosian, 2016), they consequently cause fossilized suture zones to manifest as regional high-conductivity anomalies.

Although we show this conductive metasedimentary wedge with a steep dip in Figure 2C, the exact geometry of the suture-bound conductor is not uniquely predicted by this model. The true dip of the observed anomaly will depend on the geodynamics of terrane docking. MT observations can consequently be valuable in addressing questions of subduction vergence and attitude.

Extensive empirical study has validated this relationship between suture zones and conductivity anomalies; however, there are nevertheless several requirements for the application of the genetic model shown in Figures 2A–2C.

(1) Sufficient volumes of metasedimentary material must be trapped at mid- to lower-crustal levels within a suture zone in order to produce a regional conductivity anomaly. If the passive margin or foreland basin were sediment starved before its closure, or if sedimentary units were entirely subducted before terrane docking (e.g., subduction erosion), then a conductivity anomaly may be absent. Factors that control sediment delivery, accumulation, and preservation will influence the appearance of conductivity anomalies in these settings. For example, a conductor may not appear along the suture between two juvenile volcanic arc terranes, as such terranes may not source sufficient sedimentary material (both clastic and chemical). In contrast, some of the most pronounced suture-zone conductors are associated with long-lived continental margins with well-developed passive margins (Bradley, 2008).

(2) The metasedimentary units must have the correct composition. At least some component of the metasedimentary package must have originally been deposited in a generally anoxic setting in order to host the organic carbon and/or authigenic (or diagenetic) sulfides necessary to ultimately enhance electrical conductivity; a purely quartzofeldspathic sedimentary pile is not expected to yield a conductivity anomaly. Although both ferruginous (anoxic, Fe-rich) and euxinic (anoxic, H₂S-rich) ocean water conditions will serve to limit pathways for carbon recycling and hence promote accumulation of carbon-rich sedimentary units, it is likely that euxinic conditions are most conducive to formation of the correct sediment compositions, as abundant sedimentary sulfides will also readily result from such conditions (e.g., Poulton and Canfield, 2011). Consequently, the secular evolution of ocean biogeochemistry exerts a crucial control on the formation of these conductive suture zones. Proterozoic oceans were generally conducive to the formation of the necessary sedimentary units due to widespread euxinia beneath moderately oxygenated surface waters (e.g., Canfield, 1998) and due to sufficient accumulation of organic carbon in fine-grained sedimentary rocks (e.g., Condie et al., 2001). In contrast, the correct sedimentary compositions may have been less common both in the Archean, prior to widespread euxinia in global oceans (e.g., Swanner et al., 2020; and also perhaps prior to sufficient oceanic biomass; e.g., Planavsky et al., 2021), and in the late Proterozoic and Phanerozoic, when anoxia generally and euxinia specifically became less prevalent due to global ocean oxygenation (e.g., Meyer and Kump, 2008; Swanner et al., 2020). It should be noted, however, that the degree of ocean euxinia has been spatially and temporally variable since the Archean; biogeochemical evidence indicates that oceanic basins were not uniformly euxinic in the late Paleoproterozoic and early Mesoproterozoic (e.g., Meyer and Kump, 2008; Reinhard et al., 2013; Sperling et al., 2014; Swanner et al., 2020), and euxinic conditions were sustained in certain basins at various times in the Phanerozoic (e.g., Meyer and Kump, 2008). Euxinic conditions also place requirements on basin characteristics; for example, such conditions generally required well-developed margins on the edges of major oceanic basins in the Proterozoic (Meyer and Kump, 2008; Poulton et al., 2010) and nutrient-trapping basin geometry in the Phanerozoic (Meyer and Kump, 2008). The formation of the necessary sedimentary units therefore also depends on the characteristics of the basin that was closed during terrane docking.

(3) The graphite- and/or sulfide-bearing metasedimentary units must be preserved in order to produce an observable conductivity anomaly. If the metasedimentary rocks were undisturbed and “fossilized” after initial metamorphism during terrane docking, they would expectedly produce a conductivity anomaly that could be detected with MT imaging. However, if the suture zone were significantly reactivated during subsequent tectonism, then these metasedimentary units and their associated conductivity signature may be dismembered or magmatically overprinted. Additionally, major crustal-scale fluid flow and alteration could remobilize and redistribute sulfides and graphite, thereby destroying the connectivity and hence the conductivity signature of these phases within metasedimentary units.

Violation of one of these requirements could yield an exception to the widely observed correlation between sutures and conductivity anomalies. Furthermore, the lack of a conductivity anomaly in association with a suture either must indicate that one of these requirements has not been met or may call into question the interpretation of a suture. Consequently, the observation of a lack of a colocated conductivity anomaly provides useful insights regarding the history and structure of the suture in question. It should be noted that we are unaware of any specific, convincing example of a resistive suture zone in the literature; however, the lack of any such documentation may reflect the community’s hesitancy to document “non-results” rather than a true empirical observation.

Although metasedimentary sulfides and graphite are the key phases that produce suture-bound conductivity anomalies, other basinal lithologic units, such as Superior-type banded iron formation, volcanic exhalite deposits (e.g., Algoma-type iron formation), and volcanogenic massive sulfide deposits, may also contribute to these conductivity anomalies, depending on the sequence of events preceding terrane docking (e.g., Boerner et al., 1996). Additionally, recent MT imaging work from a Cambrian-aged fossil subduction zone in Australia demonstrates that serpentinization can also leave a conductivity signature associated with a suture zone, as the magnetite that is often formed during that process may be conductive enough and well enough connected to yield a bulk conductivity anomaly (Robertson et al., 2015). It is also worth noting that these suture-bound conductors often extend to uppermost mantle depths (e.g., Jones et al., 2005). The mantle component of these anomalies is likely due to grain diminution along mantle shear zones and mantle hydration along these ancient subduction zones (e.g., Naif et al., 2021).

In contrast to the strong correlation between collisional structures and high-conductivity belts, a link between extensional structures and electrical conductivity anomalies is less ubiquitous. Active continental extension, whether through rifting or orogenic collapse, is characterized by elevated mid- to lower-crustal conductivity due to partial melt and saline fluids (e.g., Wannamaker et al., 2008; Feucht et al., 2019); however, those causative phases are not stable on long geologic time scales and consequently cannot produce conductivity anomalies ≳10 m.y. after tectonism ceases (e.g., Yardley and Valley, 1997; Manning, 2018). In Precambrian settings, there nevertheless appears to be a correlation in certain locations between extensional tectonism and high-conductivity anomalies (e.g., DeLucia et al., 2019). A likely explanation for anomalies associated with ancient extensional structures is that they are caused by graphite that precipitated from magmatically derived fluids (e.g., Huizenga, 2011; Luque et al., 2014; Fig. 2D).

There are many processes that may drive graphite precipitation from magmatic fluids (e.g., Luque et al., 2014); for example, graphite will precipitate from such fluids during cooling or during wall-rock hydration reactions. Because graphite is highly electrically conductive (e.g., Keller, 1966), just a small amount of this phase can produce a major conductivity anomaly. The carbon required to form this graphite may be scavenged and remobilized from the crust, or it may be derived from the mantle (Luque et al., 2014). Regardless of the source of carbon, magmatic flux is crucial in mobilizing carbon and producing a conductivity anomaly, and the redox state of the associated magmatic fluids directly controls both graphite precipitation and resorption (Huizenga, 2011). In many cases, the redox state of mantle-derived magmas and their associated magmatic fluids will likely be near the fayalite-magnetite-quartz oxygen fugacity buffer (Cottrell et al., 2021) and therefore generally permissive for graphite precipitation (Huizenga, 2011). However, in some settings (particularly heavily subduction-modified settings), magmatic redox conditions may be too oxidized to allow for graphite precipitation (cf. Huizenga, 2011; Cottrell et al., 2021). Furthermore, the magmatic fluid may be oxidized by crustal buffering or assimilation, and such processes could similarly inhibit graphite precipitation. Consequently, graphite precipitation from magmatic fluids may not happen in all extensional settings.

Collisional and extensional structures along terrane boundaries are often readily apparent in regional-scale MT models; however, background conductivity patterns internal to terranes can also be indicative of lithospheric architecture. The best examples of this case are found with Archean cratons, which are characterized by generally low conductivity values (<10⁻³ S/m) throughout the lithospheric column and by correspondingly low lithospheric conductance (<200 S; e.g., Bedrosian and Finn, 2021; Naif et al., 2021). The highly resistive signature of these regions reflects their geochemically depleted, refractory nature. Cratons are often, although not always, devoid of internal conductivity anomalies; where present, internal geoelectric structures are interpreted as deeply buried metasedimentary packages or as the signature of crustal magmatic-hydrothermal processes (e.g., Spratt et al., 2014; Hill et al., 2021). Such internal anomalies are generally only moderately conductive (~10⁻² S/m), and they consequently stand in sharp contrast with the high-conductivity (>10⁻¹ S/m) sutures that often border the cratons (e.g., Evans et al., 2011; Bedrosian and Finn, 2021).

In Figures 3–5, we present depth slices through an updated and expanded (cf. Kelbert et al., 2019) continent-scale electrical conductivity synthesis model as well as a vertically integrated (5–40 km depth) crustal conductance map. These images are produced by stitching together regional three-dimensional electrical conductivity models, which are derived largely from long-period MT data, via the methodology of Kelbert et al. (2019). These stitched regional models are then embedded within the global electrical conductivity model of Sun et al. (2015). Information about the constituent regional conductivity models is provided in Table 1. Additional details for two unpublished models that appear here for the first time are provided in Table 2.

The locations of individual MT sites used to construct the constituent models are shown in Figure 6. These MT data are publicly available through the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC) (https://ds.iris.edu/spud/emtf; Kelbert et al., 2011, 2018). Given our station distribution, the synthesis model at present provides good resolution for most (>80%) of the contiguous United States. A portion of the south-central United States currently lacks long-period MT data coverage; consequently, the conductivity synthesis model is at present not constrained by data in that region (Fig. 6; also Figs. 3–5, south of thick dashed line). Instead, the background model beneath these southern states is derived from sediment thickness maps (for more information, see Kelbert et al., 2019). This imposed structure reflects our expectations for the basic geoelectric characteristics of this region, but model features should not be interpreted here.

Although in places we utilize high-spatial-density MT data, in many parts of the contiguous United States the nominal spacing between MT stations is ~70 km. The lateral position of imaged conductivity structures is generally at least resolved to within that nominal station spacing. However, due to inversion regularization and the intrinsic physics of the MT method, imaged structures are also generally blurred laterally over ~30–50 km. In reality, the conductive crustal structures may be laterally constricted to a region of ≲5 km (cf. Bedrosian et al., 2018), but the nature of the MT technique results in information being smoothed laterally over a larger length scale. The vertical position of imaged conductivity anomalies is similarly not always well determined, again due to the nature of the MT technique. Here, we do not place an emphasis on the exact depth within the lithospheric column; instead, we focus generally on different segments of the lithosphere (upper crust, ~2 km depth; mid-crust, ~15 km depth; lower crust, ~35 km depth; and uppermost mantle, ~60 km depth). We expect that the general depth localization within those broad regions will be reasonably well constrained, given the data used to construct the constituent models.

Due to the nominal ~70 km MT station spacing in much of our model domain, in some cases we do not have sufficient spatial data density to accurately map the lateral continuity of structures in the mid-upper crust (depths ≲15 km). Consequently, model structures in our upper- and mid-crustal depth slices (Fig. 3) may in some places appear “patchy,” such that they visually reflect the station grid. In these “patchy” domains, the conductivity values revert to a standard background value away from data locations, with higher or lower conductivity values underneath or near the data sites. This pattern is caused by a combination of our inversion regularization approach and a lack of data sensitivity at shallow depths between data locations. This model artifact is most apparent in conductive regions; we highlight the effect of such aliasing in Figure 3A. In these “patchy” regions, a laterally continuous domain of constant conductivity would typically fit the data equally well. These artifacts have no influence on our resolution of deeper model structures.

Additionally, because this is a synthesis model, stitching artifacts exist in places along the boundaries between component model domains. Figure 6 shows the exact boundaries of constituent models, along which such artifacts may appear. These tears reflect variations in the individual component inversions (e.g., differences in regularization, starting models, applied error floors), so they should not be considered as realistic model structures and should be ignored in interpretation. An example of one of these spurious model features is noted in Figure 4A.

Here, we briefly highlight the key first-order structures that appear in our electrical conductivity synthesis model. These structures have been examined to various degrees in previous regional studies, and, due to their large-scale nature, we expect them to be well resolved in our images; consequently, these structures are robust model features. We do not explicitly discuss the detailed resolution of individual model features here, as questions of resolution have generally been well addressed in previous publications. The interested reader is directed to the references provided below.

The upper-crustal (~2 km) depth slice through the synthesis model (Fig. 3A) largely reflects the distribution of conductive Phanerozoic sedimentary rocks (cf. Fig. 1B). This image is dominated by conductive units of the Western Interior Seaway, bordered to the west and east by resistive areas of shallow or exposed basement. Other conductive areas include the Paleozoic Michigan, Illinois, and Williston basins; the Mesozoic Powder River, Denver, and Great Valley basins; and the Mesozoic–Cenozoic Atlantic Coastal Plain. The impact of the Laramide orogeny is further evident as a series of resistive basement uplifts and conductive sedimentary basins predominantly through Montana, Wyoming, and Colorado (examples noted in Fig. 3A; cf. Fig. 1B). The Phanerozoic cover conceals the Precambrian framework of Laurentia, which begins to emerge at mid-crustal (~15 km) depths (Fig. 3B), but which is most clearly imaged at lower-crustal (~35 km) and upper-mantle (~60 km) depths (Figs. 4A–4B). Here, a first-order contrast in conductivity is imaged between stable North America and the actively deforming tectonic domains of the western United States. The latter region is characterized by elevated conductivity reflecting lower-crustal melt and saline fluids beneath the Basin and Range, the Yellowstone–Snake River Plain track, and the Rio Grande Rift (Fig. 4A; e.g., Kelbert et al., 2012; Bedrosian and Feucht, 2014; Meqbel et al., 2014; Feucht et al., 2019). In contrast, mechanically stable Precambrian crustal blocks that compose the core of Laurentia stand out as coherent resistive domains.

Away from the Phanerozoic continental margins (i.e., in the U.S. Midcontinent) and below the Phanerozoic sedimentary overburden (cf. Fig. 1B), we consider our conductivity images to directly reflect Precambrian structure with essentially no Phanerozoic overprint, since Phanerozoic intracontinental tectonism (e.g., Marshak et al., 2017) was generally of insufficient degree to disturb existing conductivity structures or to create new conductivity structures. We identify a series of key model features in Figures 3–5 that define the large-scale, first-order structure of southern Laurentia and that are linked to the Precambrian architecture of this region. Figure 7 presents an abstracted sketch map that highlights these key model elements.

We recognize three highly resistive domains (average conductivity ~10⁻³ S/m, vertically integrated conductance <100 S), denoted R1, R2, and R3, that extend through the entire lithospheric column (Figs. 3–5). These resistive blocks are the geoelectric expression of the Superior craton, the Wyoming craton, and the Medicine Hat block, respectively. The influence of Cenozoic tectonomagmatism on the Wyoming craton is readily apparent on its western margin, where a zone of elevated conductivity (>10⁻¹ S/m) indicates the presence of deep saline fluids and partial melt (Fig. 4).

In the stable portion of Laurentia (particularly under North Dakota and South Dakota), these resistive domains are bordered by quasilinear, highly conductive structures (conductivity >10⁰ S/m), denoted C1 and C2, that variably span mid-crustal (~15 km) to uppermost-mantle (~60 km) depths (Figs. 3B and 4). These conductive belts, extending for over 900 km, are most evident in the conductance image (Fig. 5). They have been linked to the 1.9–1.8 Ga Trans-Hudson orogen, and they mark paleo–subduction zones of opposing polarity surrounding a deeply exhumed Paleoproterozoic arc terrane (Bedrosian and Finn, 2021). The resolvable difference in the depth of conductive belts C1 and C2 may reflect differences in the geometry and dynamics along the paleo–subduction zones that led to their formation. At their southern ends, these conductive lineaments terminate in regions we denote as C1a and C2a. These segments correspond (Yang et al., 2015; Bedrosian and Finn, 2021) to the 1.78–1.75 Ga Cheyenne belt (e.g., Chamberlain, 1998) and the ca. 1.7 Ga Spirit Lake tectonic zone (Holm et al., 2007; Van Schmus et al., 2007), respectively, which constitute the boundaries between the Archean core of Laurentia and younger Paleoproterozoic terranes to the south (e.g., Whitmeyer and Karlstrom, 2007). We also identify a broad (~200-km-wide) zone of subparallel conductive lineaments, denoted C3, between the Wyoming craton and the Medicine Hat block at lower-crustal (~35 km) depths (Fig. 4A); at least a portion of this collective structure has previously been broadly linked (Bedrosian and Feucht, 2014; Meqbel et al., 2014) to the ca. 1.86–1.71 Ga Great Falls tectonic zone (e.g., Gifford et al., 2018), which is viewed as a key structural element in the amalgamation of those two terranes (e.g., Whitmeyer and Karlstrom, 2007).

To the east of these major conductivity lineaments, we note an arcuate, east-west–trending zone of elevated conductivity (>10⁻¹ S/m), designated C4, that extends from upper-crustal (~2 km) to lower-crustal (~35 km) depths beneath Wisconsin and Michigan. This structure is most clearly imaged in the mid-crust (~15 km depth; Fig. 3B) and in the conductance image (Fig. 5), and it has been previously linked (Yang et al., 2015; Wunderman et al., 2018) to the 1.85 Ga Penokean suture in northern Wisconsin (Schulz and Cannon, 2007).

In the eastern Midcontinent, south of structure C4, we observe a set of moderate conductivity anomalies (C5; conductivity ~10⁻¹–10⁻² S/m) at mid- to lower-crustal depths that strike northwest-southeast (Figs. 3B and 4A). The largest of these anomalies (the “Missouri high-conductivity belt”) has been interpreted by DeLucia et al. (2019) as a Mesoproterozoic transtensional structure; the regional parallel anomalies likely have a similar origin (Murphy et al., 2020). High conductivity values are interpreted to be the result of graphite precipitation from magmatic fluids (DeLucia et al., 2019).

We identify a broad zone of relatively uniform, moderate lithospheric resistivity (average conductivity ~10⁻²–10⁻³ S/m, vertically integrated conductance <300 S) that extends from the region south of structure C1a and east of the conductive Rio Grande Rift (from Colorado/New Mexico) northeastward to structures C4 and C5 (into southern Wisconsin, northern Illinois, and northern Missouri). This domain, designated N1, is distinguished by its lack of major through-going conductive structures (Figs. 3–5 and 7); only locally (at spatial scales of ~100 km; e.g., in central Iowa) do conductivity values exceed 10^–2 S/m. This characteristic sharply differentiates domain N1 from regions to the north and east, which in contrast display major, large-scale conductive lineaments that extend across multiple states (at scales >100 km).

Finally, in southeastern Laurentia, our conductivity images show a pair of northeast-trending high-conductivity belts (C6) at lower-crustal (~35 km) depths that are similar in amplitude to those found in the Trans-Hudson orogen (conductivity >10⁰ S/m; Fig. 4A). Based on spatial correlations with major crustal magnetic field lineaments (Steltenpohl et al., 2010) and a mapped radiogenic isotope boundary (Fisher et al., 2010), these conductivity anomalies have been interpreted as representing a major Grenville-aged suture zone (e.g., Murphy and Egbert, 2017). In West Virginia and Ohio, at lower-crustal depths, the high-conductivity belts splay around either side of the Elzevir block (cf. Figs. 1 and 4A), which accreted during the ca. 1.25–1.2 Ga Elzevirian orogeny (e.g., Whitmeyer and Karlstrom, 2007; McLelland et al., 2010).

Our conductivity images support and expand upon existing tectonic models for the assembly of the Archean core of Laurentia (e.g., Whitmeyer and Karlstrom, 2007; Lund et al., 2015; Fig. 1), in which the constituent Archean blocks (Superior, Wyoming, Medicine Hat) are stitched together along discrete suture zones that formed during the closure of Paleoproterozoic oceanic basins. Our observations of resistive blocks R1, R2, and R3 demonstrate that these Archean cratons stand as distinct elements within the Laurentian collage. Our observations of conductive lineaments C1, C2, and C3 further highlight the nature of the Paleoproterozoic margins of these blocks; in order to produce such high-conductivity anomalies, their peripheries at the time of subduction and orogenesis must have been characterized by well-developed passive margins with basins that were conducive to accumulation of anoxic/euxinic sediments.

Similarly, our conductivity images support existing tectonic models (e.g., Whitmeyer and Karlstrom, 2007; Lund et al., 2015; Fig. 1) in which ca. 1.9 Ga Penokean crust and ca. 1.75 Ga crustal blocks (e.g., the Green Mountain arc; Chamberlain, 1998; Jones et al., 2011) are joined onto the Laurentian core along discrete suture zones (C4 and C1a/C2a, respectively). Our observations of associated suture-bound conductivity anomalies indicate that the southern edge of the amalgamated Archean core of Laurentia must have had a reasonably well-developed (and likely long-lived) passive margin in the early Paleoproterozoic, prior to the continued accretion of tectonic blocks through the Paleoproterozoic.

In southeastern Laurentia, our observed structures support existing tectonic models for Grenvillian orogenesis (e.g., Whitmeyer and Karlstrom, 2007; McLelland et al., 2010). The pair of conductive belts on either side of the Elzevir block are consistent with multiple phases of late Mesoproterozoic accretion along discrete, high-angle structures and with the closure of major basins preceding each orogenic episode. Based on the locations of these belts, the northwestern of the two conductors must represent an Elzevir-aged (ca. 1.25 Ga) suture, whereas the southeastern conductor likely formed during a subsequent phase of the greater Grenville orogenic sequence (likely the ca. 1.15 Ga Shawinigan orogeny, based on the extent of Grenvillian deformation in this portion of Laurentia; cf. McLelland et al., 2010). Additionally, the depth and attitude of these conductive structures indicate that they likely accommodated both dip-slip and strike-slip deformation during docking of exotic Precambrian basement crust onto Laurentia (Murphy and Egbert, 2017); this inference is consistent with interpretations from potential field studies (e.g., Steltenpohl et al., 2010).

Our conductivity images highlight the major boundary between Laurentian crust to the west of this suture zone, delineated by C6, and exotic crust to the east that was accreted onto Laurentia during the Grenville orogeny. Although recent passive-source seismic receiver function imaging has suggested the importance of low-angle (<10°) thrust ramps in Grenvillian orogenesis in this region (Long et al., 2019), older active-source seismic images show structures with moderate to steep dips (>30°) transecting the crust (Culotta et al., 1990). The locations of these high-angle seismically imaged structures, which have similarly been interpreted as bounding suture zones, compare favorably with our imaged conductors C6. Taken together, these geophysical observations, radiogenic isotope data (Fisher et al., 2010), and strong crustal magnetic field lineaments (Steltenpohl et al., 2010) all support a major, sharp (i.e., high-angle) crustal boundary where we observe structures C6. However, given the diversity of structural attitudes observed with various seismic techniques, further work is warranted to evaluate the timing and style of deformation along this Grenville-aged suture zone.

Although a major boundary between ca. 1.75 Ga (“Yavapai”) and ca. 1.65 Ga (“Mazatzal”) crustal provinces is drawn across much of southern Laurentia in prevailing tectonic models (e.g., Fig. 1; Holm et al., 2007; Whitmeyer and Karlstrom, 2007; Lund et al., 2015), we observe no conductive lineaments associated with a potential suture zone (cf. Figs. 1 and 3–5). The purported Yavapai-Mazatzal boundary cuts through the center of “featureless” domain N1, where we observe no major through-going conductivity anomalies. We consider three possible explanations for the lack of conductive lineaments through this region, which each have clear implications for the history of the constituent Precambrian crust.

• (1) Although Paleoproterozoic ocean biogeochemistry was globally conducive to the formation of euxinic sedimentary deposits (e.g., Meyer and Kump, 2008; Swanner et al., 2020), the basin(s) between the Yavapai and Mazatzal provinces may not have had a geometry conducive to formation of such deposits. The marginal basins along the juvenile volcanic arc terranes that are generally viewed as composing these provinces (e.g., Whitmeyer and Karlstrom, 2007) may not have been well enough developed, in terms of both geometry and temporal duration, to accumulate sufficient sediment of the correct composition to form a suture-bound conductivity anomaly.

• (2) Major late-Paleoproterozoic to early-Mesoproterozoic intracontinental tectonomagmatism (e.g., Whitmeyer and Karlstrom, 2007; Duebendorfer et al., 2015) may have destroyed the electrical signature of any suture within domain N1, if tectonomagmatic activity was widespread throughout southern Laurentia. The “gap” that we observe in the ca. 1.8–1.7 Ga suture between the Cheyenne belt and the Spirit Lake tectonic zone (separating C1a from C2a), for example, may be the result of a post-orogenic batholith, inferred from gravity data, that intruded this segment of the suture (Bedrosian and Finn, 2021). A possible discrete Yavapai-Mazatzal suture may consequently have been erased from the geologic record by the “stitching” magmatism that is a key component of prevailing tectonic models for Paleoproterozoic–Mesoproterozoic accretion (e.g., Whitmeyer and Karlstrom, 2007) or by the ca. 1.4 Ga Granite-Rhyolite event (e.g., Bickford et al., 2015).

• (3) Laurentia may have never experienced a discrete margin-wide Yavapai-Mazatzal orogenic suturing event; instead, these domains may be better characterized by the continuous accretion of small, diachronous blocks throughout the late Paleoproterozoic (and into the early Mesoproterozoic; e.g., Daniel et al., 2013; Aronoff et al., 2016), with additional crustal evolution through continental arc growth and collapse as well as back-arc basin expansion and contraction (e.g., Condie, 1982). Individual oceanic arcs or microterranes would be expected to incorporate very little conductive sedimentary material along their limited short-lived margins, and the basins associated with continental arcs would likely also host little conductive sedimentary material, so the boundaries between these small individual tectonic elements would leave no crustal-scale conductivity signature. The amalgamated Yavapai and Mazatzal “provinces” as often drawn are then misleading, as they lack clear crustal boundaries (cf. the Trans-Hudson orogen) and incorporate broadly coeval (i.e., ca. 1.8–1.7 Ga, ca. 1.7–1.6 Ga) but disparate zones across much of southern Laurentia that may have substantially different histories along strike. In this case, the pieces and processes matter far more than the end result as a whole in explaining the observed structure and the style of Laurentian growth.

These three explanations are not necessarily mutually exclusive, and they may in fact operate together to explain the lack of through-going conductivity anomalies. Regardless, current prevailing tectonic models are insufficient to explain the observed electrical conductivity structure of these late Paleoproterozoic basement domains. Our observations particularly favor new tectonic models that stress the progressive growth of southern Laurentia through continental margin processes (e.g., repeated growth and collapse of marginal volcanic arcs; Condie, 1982; Jones et al., 2010) rather than accretion, as a lack of discrete suturing events can rigorously explain the “featureless” region N1. It is worth noting that the key difference between the electrical signatures of the ca. 1.8–1.7 Ga Cheyenne belt and Spirit Lake tectonic zone and any convergent structures within the ca. 1.75 Ga and ca. 1.65 Ga crustal provinces that fall within region N1 may be the degree of passive-margin development. The Cheyenne belt and Spirit Lake tectonic zone likely closed a major ocean basin that was conducive to accumulation of euxinic sedimentary deposits; a lack of prolonged margin development after formation of those structures could alternatively explain the lack of major conductivity lineaments.

Mesozoic–Cenozoic tectonomagmatic overprinting of Precambrian electrical signatures in Arizona and New Mexico (Figs. 3–5) appears to have eliminated any hope of tracing electrical structures from the southwestern United States, where the inferred Yavapai-Mazatzal boundary is exposed at the surface (e.g., Karlstrom et al., 1987), into the enigmatic U.S. Midcontinent with the goal of deciphering through-going crustal domains. However, targeted MT experiments may be able to track down the remaining electrical signatures of proposed Paleoproterozoic structures at upper- and mid-crustal levels, where they may not have been entirely destroyed by recent tectonomagmatic activity. An effective strategy to study these late Paleoproterozoic basement terranes across southern Laurentia may then be to collect focused, high-frequency (broadband) MT data in several key areas in order to compare how upper-/mid-crustal electrical structure changes along strike of an inferred boundary.

Due to poor Precambrian basement exposure in the eastern and southern U.S. Midcontinent, radiogenic isotope data from sparse basement-penetrating boreholes have been used to define a major Sm-Nd isotopic boundary, commonly called the “Nd line” (Fig. 1), that separates crust characterized by magmatic rocks with Sm-Nd depleted-mantle model ages (T_DM) >1.55 Ga to the northwest from crust characterized by magmatic rocks with T_DM <1.55 Ga to the southeast (Fig. 1; Bickford et al., 2015; Ayuso et al., 2016). Prevailing tectonic models infer that this boundary represents a cryptic (i.e., poorly expressed) early Mesoproterozoic (ca. 1.55 Ga) suture (Whitmeyer and Karlstrom, 2007; Bickford et al., 2015; Lund et al., 2015). However, the isotopic data that delineate this boundary are best defined in Texas; Oklahoma; Missouri; and southern Ontario, Canada. Almost no data constrain the boundary through Illinois, Indiana, and Ohio, where the conductivity synthesis model in contrast provides valuable information. Although there is some lithospheric magnetic field evidence to support a contrast in regional basement properties along the Nd line (Ravat, 2007; McCafferty et al., 2019), our conductivity images show no clear evidence for a suture zone in this region. Instead, our observed structures C5 cut nearly perpendicular to this inferred boundary.

Accretion along a cryptic boundary in this region may not have met the conditions necessary for a suture-bound conductivity anomaly, or subsequent tectonomagmatism could have obliterated the electrical signature of any suture associated with the Nd line. However, recent Lu-Hf data from the eastern U.S. Midcontinent (Petersson et al., 2015) demonstrate that, at least in places, Paleoproterozoic basement extends eastward past the Nd line. Based on consideration of other geophysical data and petrologic data from Mesoproterozoic magmatic rocks exposed in southeastern Missouri, structures C5 have been interpreted to represent transtensional deformation along the Mesoproterozoic edge of Laurentia (DeLucia et al., 2019; Murphy et al., 2020), with high conductivity values attributable to graphite precipitated from magmatic fluids. Consequently, unlike in the Picuris orogen to the southwest (Daniel et al., 2013; Aronoff et al., 2016) and the Baraboo orogen to the north (Medaris et al., 2021), there may not have been a Mesoproterozoic convergent boundary in this portion of Laurentia. Taken together, the Lu-Hf data and our conductivity observations indicate that alternative models for crustal evolution in this region must be explored. Although the age of structures C5 is at present only loosely constrained to be Precambrian, we consider it likely that these structures may have served to segment deformation along the Mesoproterozoic Laurentian margin, thereby linking Picuris and Baraboo orogenesis during the simultaneous emplacement of ferroan (“anorogenic”) magmas during the ca. 1.4 Ga Granite-Rhyolite event (e.g., Daniel et al., this volume). These transtensional structures in particular may have served as conduits for mantle-derived mafic melts that drove that voluminous magmatism.

We observe no major conductivity structures that are clearly associated with either the Picuris orogeny (Daniel et al., 2013; Aronoff et al., 2016) or the Baraboo orogeny (Medaris et al., 2021). The eastern boundaries of the Picuris orogeny and the general location of the Baraboo orogeny fall within region N1 (cf. Figs. 1 and 7), where we observe no major through-going conductivity structures. Although further careful consideration of electrical structures in the context of these orogenic events is warranted, our observations here support the view that these orogenies largely involved crustal growth and deformation within a dynamic continental margin arc system, without closure of large oceanic basins and docking of new terranes along major subduction zones (e.g., Daniel et al., 2013).

We have presented an updated and refined continent-scale electrical conductivity model derived largely from long-period MT data, and we have discussed implications of the large-scale, first-order structures that are readily apparent within the model. Two key conclusions arise from this discussion.

• (1) The high-conductivity belts associated with the Trans-Hudson, Penokean, and Grenville sutures demonstrate that these accretionary events entailed the closure of well-developed oceanic basins and the stitching of terranes along discrete structures. The conductive lineaments along ca. 1.8–1.7 Ga accretionary structures (the Cheyenne belt and the Spirit Lake tectonic zone) similarly demonstrate that the southern margin of Laurentia featured a well-developed and well-defined passive margin before the onset of late Paleoproterozoic to early Mesoproterozoic (ca. 1.8–1.4 Ga) accretion. Our observations here support and reinforce existing tectonic models.

• (2) In contrast, there are no clear continent-scale conductivity anomalies associated with the traditionally interpreted late Paleoproterozoic to early Mesoproterozoic (ca. 1.8–1.4 Ga) accretionary boundaries (e.g., the purported Yavapai-Mazatzal suture). Our MT images indicate that the southern Laurentian margin was not conducive to formation and/or preservation of euxinic (i.e., conductive) metasedimentary units during this time, due to marginal basin characteristics or to processes operating on the margin; that a major subsequent episode of widespread intracontinental deformation and magmatism destroyed the geoelectric signatures of the associated suture(s); and/or that southward continental growth during this time did not proceed via the accretion of coherent tectonic blocks along discrete boundaries. Our conductivity images challenge existing tectonic models here, and they provide constraints in formulating new models for this time period.

More broadly, our images provide a valuable source of information for developing and refining new tectonic models of Precambrian Laurentian assembly and evolution. Particularly, we expect that our synthesis model will provide useful constraints in integrating the early Mesoproterozoic Picuris (e.g., Daniel et al., 2013) and Baraboo (e.g., Medaris et al., 2021) orogenies into broader frameworks of Paleoproterozoic and Mesoproterozoic tectonics in southern Laurentia.

The electrical conductivity model that we present here is publicly available via the IRIS Earth Model Collaboration (EMC; https://ds.iris.edu/ds/products/emc/). Conversations with Jamey Jones, Bob Gaines, Steve Marshak, Mike DeLucia, and Tiku Ravat greatly benefited this manuscript. We thank Ruth Aronoff, Daniel Holm, Kevin Mendoza, an anonymous reviewer, and Volume Editor Basil Tikoff for providing valuable feedback on this manuscript. We also thank Jamey Jones, Krissy Lewis, Brian Shiro, and Janet Slate for internal reviews of this manuscript. B.S. Murphy was supported by a Mendenhall Postdoctoral Fellowship through the U.S. Geological Survey.